1. Field of the Invention
The present invention relates to light beam scanning apparatus, and more particularly relates to apparatus for detecting defects in information discs such as optical discs.
2. Brief Description of the Prior Art
An optical disc is a clear plastic disc approximately the size of a standard LP phonograph record, which has a thin reflective layer imbedded in its interior. The reflective layer is provided with a spiral-shaped information track in the form of a series of spaced-apart indicia, such as pits or holes, which can be "read" by a focused laser beam by directing the focused spot of the laser beam along the spiral-shaped track and detecting the variations in the intensity of the light reflected therefrom as the spot encounters alternately indicia and the reflective surface between the indicia. A detailed description of such a disc, in which the indicia are holes may be found in U.S. Pat. No. 4,264,911 which issued on Apr. 28, 1981 to Richard L. Wilkinson.
The size and spacing of the indicia, and the spacing of adjacent spiral tracks in a typical optical disc are extremely small, being of the order of magnitude of approximately one micron. In order to "read" the information contained in the indicia of such extremely small dimensions, the reading laser must be focused to an extremely small spot, being also of the order of magnitude of one micron.
Hole-type indicia can be formed in the reflective layer by means of a high intensity "writing" laser which is focused onto the layer and turned on and off as it it directed across the disc in spiral fashion. When the writing laser is turned on, it causes a hole to form in the reflective layer where it is focused, and when it is turned off it leaves the layer unaffected. Information can be imparted to the track by controlling the length and spacing of the holes.
In reading optical discs, whether of the "hole" type or the "pit" type, the reflected light of the reading laser is directed to a photo detector where the variation in intensity of the light incident on the photo detector due to the presence and absence of indicia in the reading track cause the output signal of the photo detector to take the form of a series of pulses. The length and spacing in time of these pulses correspond to the length and spacing of the indicia on the optical disc, and in this way the recorded information is recovered.
In both reading and writing it is necessary to maintain the laser spot in focus on the surface of the reflective layer. In both operations this is accomplished typically by means of a separate servo system which controls the position of an objective lens over the surface of the disc.
A description of an optical disc player including a disclosure of a technique for maintaining a laser beam focused to an appropriately small spot on an optical disc as it is "played" on the device may be found in U.S. Pat. No. 4,332,022, issued May 25, 1982, and commonly assigned to the assignee of the present invention.
The player apparatus disclosed in that application is designed especially to play video optical discs that contain video information for storage and playback of television program material. Briefly, the technique disclosed therein for maintaining the focus of the spot of laser light on the disc involves the provision of a separate beam of laser light substantially parallel to the reading laser light beam. This separate laser beam is focused onto the disc by the same optical system used to focus the read beam. The light of the separate laser beam is reflected off of the laser disc and directed to a special photo detector which detects the size of the beam incident thereon and provides an output signal indicative of the state of focus of the separate light beam. This output signal is used in a focus servo mechanism to maintain the separate beam in a focused condition even if the disc is warped. This also maintains the read beam in focus.
Such a focus servo system is used in writing apparatus as well.
It has been discovered, however, that the presence of certain kinds of defects in the reflective surface of the video optical disc, namely "dimples" and "inclusions", can cause the focus servo system to exceed its limits and thereby cause the optical system to lose focus, with consequent loss of information storage or retrieval, as the case may be. In the case of video disc playback, this can result in the temporary disruption of the video picture, frame skipping, and jump back, all of which are unacceptable from a commercial standpoint.
Since such defects are typically too small to be seen with the naked eye, visual inspection is an inadequate technique. In addition, not all defects give rise to disruptions of an unacceptable nature. Consequently, the technique utilized in the commercial production of optical video discs involves simply playing all or part of the disc and looking for any disturbances in the video picture which are considered unacceptable. However, if the entire disc is played, this is highly costly in man-hours. If only a part of the disc is played, however, not all defects will be detected.
It will therefore be appreciated that there is a need for an apparatus for detecting defects in optical discs which permits the rapid identification and location of defects in conjunction with a player apparatus to enable the rapid and selective playing of only those areas of the disc suspected of being defective to determine if an unacceptable defect exists at that area.
In the case of computer data storage and retrieval, if a defect is known to exist on the reflective layer of an optical disc, a special code can be inserted into the information track immediately in advance of the defect which will cause the information reading apparatus to stop looking for information for the period of time it takes the read beam to pass over the defect. Data retrieval can then be resumed, and in this way no information is lost. Thus, optical discs having defects can be used to store computer data for retrieval if the location and size of defects can be accurately determined in conjunction with an apparatus for storing the information on the disc.
It will therefore be appreciated there is is a need for a defect detecting apparatus which provides accurate indentification of the location and size of defects in optical discs. Further, there is a need for a defect detector apparatus which provides accurate detection of defects in conjunction with a laser writing function for an optical disc to permit the precise localization of the writing laser beam in advance of the defective area.
The present invention fulfills the aforementioned needs.